IPNAT(5) IPNAT(5)
NAME
ipnat, ipnat.conf - IP NAT file format
DESCRIPTION
The format for files accepted by ipnat is described by the following
grammar:
ipmap :: = mapblock | redir | map .
map ::= mapit ifname lhs "->" dstipmask [ mapicmp | mapport | mapproxy ]
mapoptions .
mapblock ::= "map-block" ifname lhs "->" ipmask [ ports ] mapoptions .
redir ::= "rdr" ifname rlhs "->" ip [ "," ip ] rdrport rdroptions .
lhs ::= ipmask | fromto .
rlhs ::= ipmask dport | fromto .
dport ::= "port" portnum [ "-" portnum ] .
ports ::= "ports" numports | "auto" .
rdrport ::= "port" portnum .
mapit ::= "map" | "bimap" .
fromto ::= "from" object "to" object .
ipmask ::= ip "/" bits | ip "/" mask | ip "netmask" mask .
dstipmask ::= ipmask | "range" ip "-" ip .
mapicmp ::= "icmpidmap" "icmp" number ":" number .
mapport ::= "portmap" tcpudp portspec .
mapoptions ::= [ tcpudp ] [ "frag" ] [ age ] [ clamp ] .
rdroptions ::= rdrproto [ rr ] [ "frag" ] [ age ] [ clamp ] [ rdrproxy ] .
object :: = addr [ port-comp | port-range ] .
addr :: = "any" | nummask | host-name [ "mask" ipaddr | "mask" hexnumber ] .
port-comp :: = "port" compare port-num .
port-range :: = "port" port-num range port-num .
rdrproto ::= tcpudp | protocol .
rr ::= "round-robin" .
age ::= "age" decnumber [ "/" decnumber ] .
clamp ::= "mssclamp" decnumber .
tcpudp ::= "tcp/udp" | protocol .
mapproxy ::= "proxy" "port" port proxy-name '/' protocol
rdrproxy ::= "proxy" proxy-name .
protocol ::= protocol-name | decnumber .
nummask ::= host-name [ "/" decnumber ] .
portspec ::= "auto" | portnumber ":" portnumber .
port ::= portnumber | port-name .
portnumber ::= number { numbers } .
ifname ::= 'A' - 'Z' { 'A' - 'Z' } numbers .
numbers ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' .
For standard NAT functionality, a rule should start with map and then
proceeds to specify the interface for which outgoing packets will have
their source address rewritten.
Packets which will be rewritten can only be selected by matching the
original source address. A netmask must be specified with the IP
address.
The address selected for replacing the original is chosen from an
IP#/netmask pair. A netmask of all 1's indicating a hostname is valid.
A netmask of 31 1's (255.255.255.254) is considered invalid as there is
no space for allocating host IP#'s after consideration for broadcast
and network addresses.
When remapping TCP and UDP packets, it is also possible to change the
source port number. Either TCP or UDP or both can be selected by each
rule, with a range of port numbers to remap into given as port-num-ber:port-number.
COMMANDS
There are four commands recognised by IP Filter's NAT code:
map that is used for mapping one address or network to another in an
unregulated round robin fashion;
rdr that is used for redirecting packets to one IP address and port
pair to another;
bimap for setting up bidirectional NAT between an external IP address
and an internal IP address and
map-block
which sets up static IP address based translation, based on a
algorithm to squeeze the addresses to be translated into the
destination range.
MATCHING
For basic NAT and redirection of packets, the address subject to change
is used along with its protocol to check if a packet should be altered.
The packet matching part of the rule is to the left of the "->" in each
rule.
Matching of packets has now been extended to allow more complex com-
pares. In place of the address which is to be translated, an IP
address and port number comparison can be made using the same expres-
sions available with ipf. A simple NAT rule could be written as:
map de0 10.1.0.0/16 -> 201.2.3.4/32
or as
map de0 from 10.1.0.0/16 to any -> 201.2.3.4/32
Only IP address and port numbers can be compared against. This is
available with all NAT rules.
TRANSLATION
To the right of the "->" is the address and port specification which
will be written into the packet providing it has already successfully
matched the prior constraints. The case of redirections (rdr) is the
simplest: the new destination address is that specified in the rule.
For map rules, the destination address will be one for which the tuple
combining the new source and destination is known to be unique. If the
packet is either a TCP or UDP packet, the destination and source ports
come into the equation too. If the tuple already exists, IP Filter
will increment the port number first, within the available range speci-
fied with portmap and if there exists no unique tuple, the source
address will be incremented within the specified netmask. If a unique
tuple cannot be determined, then the packet will not be translated.
The map-block is more limited in how it searches for a new, free and
unique tuple, in that it will used an algorithm to determine what the
new source address should be, along with the range of available ports -
the IP address is never changed and nor does the port number ever
exceed its allotted range.
ICMPIDMAP
ICMP messages can be divided into two groups: "errors" and "queries".
ICMP errors are generated as a response of another IP packet. IP Filter
will take care that ICMP errors that are the response of a NAT-ed IP
packet are handled properly.
For 4 types of ICMP queries (echo request, timestamp request, informa-
tion request and address mask request) IP Filter supports an additional
mapping called "ICMP id mapping". All these 4 types of ICMP queries use
a unique identifier called the ICMP id. This id is set by the process
sending the ICMP query and it is usually equal to the process id. The
receiver of the ICMP query will use the same id in its response, thus
enabling the sender to recognize that the incoming ICMP reply is
intended for him and is an answer to a query that he made. The "ICMP id
mapping" feature modifies these ICMP id in a way identical to portmap
for TCP or UDP.
The reason that you might want this, is that using this feature you
don't need an IP address per host behind the NAT box, that wants to do
ICMP queries. The two numbers behind the icmpidmap keyword are the
first and the last icmp id number that can be used. There is one impor-
tant caveat: if you map to an IP address that belongs to the NAT box
itself (notably if you have only a single public IP address), then you
must ensure that the NAT box does not use the icmpidmap range that you
specified in the map rule. Since the ICMP id is usually the process
id, it is wise to restrict the largest permittable process id (PID) on
your operating system to e.g. 63999 and use the range 64000:65535 for
ICMP id mapping. Changing the maximal PID is system dependent. For most
BSD derived systems can be done by changing PID_MAX in
/usr/include/sys/proc.h and then rebuild the system.
KERNEL PROXIES
IP Filter comes with a few, simple, proxies built into the code that is
loaded into the kernel to allow secondary channels to be opened without
forcing the packets through a user program. The current state of the
proxies is listed below, as one of three states:
Aging - protocol is roughly understood from the time at which the proxy
was written but it is not well tested or maintained;
Developmental - basic functionality exists, works most of the time but
may be problematic in extended real use;
Experimental - rough support for the protocol at best, may or may not
work as testing has been at best sporadic, possible large scale
changes to the code in order to properly support the protocol.
Mature - well tested, protocol is properly understood by the proxy;
The currently compiled in proxy list is as follows:
FTP - Mature
IRC - Experimental
rpcbind - Experimental
H.323 - Experimental
Real Audio (PNA) - Aging
IPsec - Developmental
netbios - Experimental
R-command - Mature
TRANSPARENT PROXIES
True transparent proxying should be performed using the redirect (rdr)
rules directing ports to localhost (127.0.0.1) with the proxy program
doing a lookup through /dev/ipnat to determine the real source and
address of the connection.
LOAD-BALANCING
Two options for use with rdr are available to support primitive, round-robin based load balancing. The first option allows for a rdr to spec-
ify a second destination, as follows:
rdr le0 203.1.2.3/32 port 80 -> 203.1.2.3,203.1.2.4 port 80 tcp
This would send alternate connections to either 203.1.2.3 or 203.1.2.4.
In scenarios where the load is being spread amongst a larger set of
servers, you can use:
rdr le0 203.1.2.3/32 port 80 -> 203.1.2.3,203.1.2.4 port 80 tcp round-robin
rdr le0 203.1.2.3/32 port 80 -> 203.1.2.5 port 80 tcp round-robin
In this case, a connection will be redirected to 203.1.2.3, then
203.1.2.4 and then 203.1.2.5 before going back to 203.1.2.3. In accom-
plishing this, the rule is removed from the top of the list and added
to the end, automatically, as required. This will not effect the dis-
play of rules using "ipnat -l", only the internal application order.
EXAMPLES
This section deals with the map command and its variations.
To change IP#'s used internally from network 10 into an ISP provided 8
bit subnet at 209.1.2.0 through the ppp0 interface, the following would
be used:
map ppp0 10.0.0.0/8 -> 209.1.2.0/24
The obvious problem here is we're trying to squeeze over 16,000,000 IP
addresses into a 254 address space. To increase the scope, remapping
for TCP and/or UDP, port remapping can be used;
map ppp0 10.0.0.0/8 -> 209.1.2.0/24 portmap tcp/udp 1025:65000
which falls only 527,566 `addresses' short of the space available in
network 10. If we were to combine these rules, they would need to be
specified as follows:
map ppp0 10.0.0.0/8 -> 209.1.2.0/24 portmap tcp/udp 1025:65000
map ppp0 10.0.0.0/8 -> 209.1.2.0/24
so that all TCP/UDP packets were port mapped and only other protocols,
such as ICMP, only have their IP# changed. In some instances, it is
more appropriate to use the keyword auto in place of an actual range of
port numbers if you want to guarantee simultaneous access to all within
the given range. However, in the above case, it would default to 1
port per IP address, since we need to squeeze 24 bits of address space
into 8. A good example of how this is used might be:
map ppp0 172.192.0.0/16 -> 209.1.2.0/24 portmap tcp/udp auto
which would result in each IP address being given a small range of
ports to use (252). In all cases, the new port number that is used is
deterministic. That is, port X will always map to port Y. WARNING: It
is not advisable to use the auto feature if you are map'ing to a /32
(i.e. 0/32) because the NAT code will try to map multiple hosts to the
same port number, outgoing and ultimately this will only succeed for
one of them. The problem here is that the map directive tells the NAT
code to use the next address/port pair available for an outgoing con-
nection, resulting in no easily discernible relation between external
addresses/ports and internal ones. This is overcome by using map-block
as follows:
map-block ppp0 172.192.0.0/16 -> 209.1.2.0/24 ports auto
For example, this would result in 172.192.0.0/24 being mapped to
209.1.2.0/32 with each address, from 172.192.0.0 to 172.192.0.255 hav-
ing 252 ports of its own. As opposed to the above use of map, if for
some reason the user of (say) 172.192.0.2 wanted 260 simultaneous con-
nections going out, they would be limited to 252 with map-block but
would just move on to the next IP address with the map command.
FILES
/dev/ipnat
/etc/services
/etc/hosts
/usr/share/examples/ipf Directory with examples.
SEE ALSOipnat(4), hosts(5), ipf(5), services(5), ipf(8), ipnat(8)IPNAT(5)

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